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    <script>
        /*  
          +
            可以将字符串转化为Number类型的正数

          -
            可以将字符串转化为Number类型的负数
        */
       var a = "123";
       alert(typeof +a);
       //结果：number
       
       var b = "456";
       alert(-b);
       //结果：-456    
    </script>
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